+1(514) 937-9445 or Toll-free (Canada & US) +1 (888) 947-9445
You cannot even board the plane if you don't have a visa - otherwise the airline will be fined. And no, you cannot print your boarding pass either, because you carry a passport that requires a visa to enter Canada. The machine will not print the boarding pass for you, and will instead alert the attendant who will need to come by the machine, check your passport to make sure you have a valid visa, and only after then will you be issued a boarding pass.
 
I flew to Canada in July, and I had only a passport, and no Visa. For United States citizens, you can stay legally up to 6 months without needing to apply for any visas. I had no trouble at the border, I simply presented my passport and told them what I was doing in Canada. If you aren't a US citizen though, you'll need to apply for a visitor visa. I don't think they take long. You say what you want to do, how long you want to stay, and prove you have funds to support yourself and plan on leaving at the end of your legal stay. Then pay the fees, and you'll probably have it in less than 2 weeks. When is your flight? I'd postpone it and be legal, as if you are unsure, Canada won't feel bad if they don't let you in. And the airline will not refund you.
 
the magic words are "United States citizens"
 
I second Scylla here. Without proper documentation, the airline shouldn't allow you on the flight. If you get a hawk-eyed agent at security, you might not even make it to the counter. My first trip to Canada I presented my drivers license and boarding pass at security. The agent asked for my passport and kindly explained I would need that instead of my license at all checkpoints even on the USA side. (I felt like a newbie! And I was lol!)

There's multiple points you'll be stopped. Chances of getting through all those points without proper travel documents is virtually 0. If this is a true emergency, calling the consulate might be a better idea.